Machinery and vehicles of almost all type can pose significant threats to the well-being of persons in their vicinity. Particularly dangerous areas include places where loud, heavy, mobile machinery are present such as construction sites and on airplane tarmacs or launching areas (e.g., on an aircraft carrier). In many cases, workers within such sites may wear protective hearing devices, either out of good practice or as mandated by their employer or a federal protection agency such as OSHA. While hearing protection devices can perform well for their intended function, in some cases they can remove one of the critical senses humans rely upon in perceiving danger.
Another particularly dangerous environment is one where threats are visually obscured or substantially invisible. For example, the spinning propeller of an airplane or the inlet side of a turbine jet engine can present a serious threat to anyone in the vicinity. In the former case, propellers can spin quickly enough that the blades become difficult to see, and in the latter case, turbine jet engines are capable of producing suction powerful enough to draw a person into the inlet. Such dangers are especially prevalent at night and in adverse weather conditions when perception can be further hindered.